I. Field of the Invention
The present invention relates generally to radiotelephone communications. More particularly, the present invention relates to reverse link power control in a radiotelephone system.
II. Description of the Related Art
The Federal Communications Commission (FCC) governs the use of the radio frequency (RF) spectrum, deciding which industry gets certain frequencies. Since the RF spectrum is limited, only a small portion of the spectrum can be assigned to each industry. The assigned spectrum, therefore, must be used efficiently in order to allow as many frequency users as possible to have access to the spectrum.
Multiple access modulation techniques are some of the most efficient techniques for utilizing the RF spectrum. Examples of such modulation techniques include time division multiple access (TDMA), frequency division multiple access (FDMA), and code division multiple access (CDMA).
CDMA modulation employs a spread spectrum technique for the transmission of information. A spread spectrum system uses a modulation technique that spreads the transmitted signal over a wide frequency band. This frequency band is typically substantially wider than the minimum bandwidth required to transmit the signal. The spread spectrum technique is accomplished by modulating each baseband data signal to be transmitted with a unique wide band spreading code. Using this technique, a signal having a bandwidth of only a few kilohertz can be spread over a bandwidth of more than a megahertz. Typical examples of spread spectrum techniques can be found in Spread Spectrum Communications, Volume i, M. K. Simon, Chap. 5, pp. 262-358.
A form of frequency diversity is obtained by spreading the transmitted signal over a wide frequency range. Since only 200-300 kHz of a signal is typically affected by a frequency selective fade, the remaining spectrum of the transmitted signal is unaffected. A receiver that receives the spread spectrum signal, therefore, will be affected less by the fade condition.
In a CDMA-type radiotelephone system, multiple signals are transmitted simultaneously at the same frequency. Such a CDMA system is disclosed in U.S. Pat. No. 4,901,307 to Gilhousen et al. and assigned to Qualcomm, Inc. In this type system, a particular receiver determines which signal is intended for that receiver by the unique spreading code in the signal. The signals at that frequency without the particular spreading code intended for that particular receiver appear to be noise to that receiver and are ignored.
FIG. 1 shows a typical prior art CDMA transmitter for use on the reverse channel of a radiotelephone system, the reverse channel being the link from the mobile to the base station. A digital baseband signal is first generated by a vocoder (voice encoder/decoder). The vocoder (100) digitizes an analog voice or data signal using an encoding process such as the Code Excited Linear Prediction (CELP) process that is well known in the art.
The digital baseband signal is input to a convolutional encoder (101) at a particular rate, such as 9600 bps. The encoder (101) convolutionally encodes the input data bits into data symbols at a fixed encoding rate. For example, the encoder (101) could encode the data bits at a fixed encoding rate of one data bit to three data symbols such that the encoder (101) outputs data symbols at a 28.8 ksym/s rate with a 9600 bps input rate.
The data symbols from the encoder are input to an interleaver (102). The interleaver (102) scrambles the symbols such that any symbols lost over the channel won't be contiguous symbols. Therefore, if more than one symbol is lost in the communications channel, the error correcting code is able to recover the information. The data symbols are input into the interleaver (102) in a column by column matrix and output from the matrix row by row. The interleaving takes place at the same 28.8 ksym/s data symbol rate that the data symbols were input.
The interleaved data symbols are input to a modulator (104). The modulator (104) derives a sequence of fixed length Walsh symbols from the interleaved data symbols. In 64-ary orthogonal code signaling, the interleaved data symbols are grouped into sets of six to select one out of the 64 orthogonal codes to represent the set of six data symbols. These 64 orthogonal codes correspond to Walsh symbols from a 64 by 64 Hadamard matrix wherein a Walsh symbol is a single row or column of the matrix. The modulator outputs a sequence of Walsh symbols, corresponding to the input data symbols at a fixed symbol rate, to one input of an XOR combiner (107). The set of six grouped Walsh symbols has a length of 1.25 milliseconds (ms) and is typically referred to as a power control group.
A pseudo random noise (PN) generator (103) uses a long PN sequence to generate a user specific sequence of symbols. In a mobile radio-telephone having an electronic serial number (ESN), the ESN can be exclusive-ORed with the long PN sequence to generate the sequence, making the sequence specific to that radiotelephone user. The long PN generator (103) inputs and outputs data at the spreading rate of the system. The output of the PN generator (103) is coupled to the XOR combiner (107).
The Walsh code spread symbols from the combiner (107) are next spread in quadrature. The symbols are input to two XOR combiners (108 and 109) that generate a pair of short PN sequences. The first combiner (108) XORs the Walsh code spread symbols with the in-phase (I) sequence (105) while the second combiner (109) XORs the Walsh code spread symbols with the quadrature phase (Q) sequence (106).
The resulting I and Q channel code spread sequences are used to biphase modulate a quadrature pair of sinusoids by driving the power level of the pair of sinusoids. The sinusoidal output signals are then summed, bandpass filtered, translated to an RF frequency, amplified, filtered, and radiated by an antenna.
The typical prior art CDMA transmitter used on the forward channel of a radiotelephone system, the link from the base station to the mobile, is similar to the reverse channel. This transmitter is illustrated in FIG. 2. The difference between the forward and reverse channel transmitters is the addition of a Walsh code generator (201) and power control bit multiplexer (220) between the PN generator combiner (103) and the quadrature spreading combiners (108 and 109) for the forward channel transmitter.
The power control bit multiplexer (220) multiplexes a power control bit in place of another bit in the frame. The mobile knows the location of this bit and looks for this power control bit at that location. As an example, a "0" bit instructs the mobile to increase its mean output power level a predetermined amount and a "1" bit instructs the mobile to decrease its mean output level a predetermined amount.
The code division channel selection generator (201) is coupled to a combiner (202) and provides a particular Walsh symbol to the combiner (202). The generator (201) provides one of 64 orthogonal codes corresponding to 64 Walsh symbols from a 64 by 64 Hadamard matrix wherein a Walsh symbol is a single row or column of the matrix. The combiner (202) uses the particular Walsh code input by the code division channel generator (201) to spread the input scrambled data symbols into Walsh code spread data symbols. The Walsh code spread data symbols are output from the XOR combiner (202) and into the quadrature spreading combiners at a fixed chip rate of 1.2288 Mchp/s.
In the previously described system, as the mobile's transmission bit rate is reduced, it is desirable to reduce the average transmitter power accordingly. The mobile, therefore, reduces its transmit power, by reducing its transmitter duty cycle, as the data rate decreases. This permits the base station to measure the mobile's received signal to noise ratio (SNR) in each 1.25 ms. interval of six Walsh symbols, also known in the art as a power control group, and comparing this with a constant standard without the need to know the actual transmission rate being utilized in each data frame.
Each 20 ms. long data frame is comprised of 16 power control groups. Co-pending patent application U.S. Ser. No. 07/822,164 to Padovani et al. and assigned to Qualcomm, Inc. recites a more detailed explanation of the 20 ms frames that are transmitted on the forward and reverse channels. The amount of data transmitted in each frame depends on the data rate. The frame composition for each data rate for the forward and reverse channels is illustrated in the following table:
______________________________________ Raw # bits CRC Tail Rsrvd Info bit Rate ______________________________________ 288 12 8 3 265 13250 144 10 8 2 124 6200 72 8 8 2 54 2700 36 6 8 2 20 1000 ______________________________________
The rate listed in the table is the information bit rate. The reserved bits for the forward and reverse channels, in the preferred embodiment, are for signaling, power control, and future use.
During each power control group that the mobile is transmitting, it transmits at a power level determined by the power control system of the base station. The base station measures the received SNR of each received mobile signal during the 1.25 ms. power control interval and compares it to a target SNR established for that particular mobile. If the SNR exceeds the target SNR, a "turn down" command is transmitted from the base station to the mobile. Otherwise a "turn up" command is sent.
These power control commands are transmitted to the mobile by puncturing the data transmission with the power control bit. This puncturing replaces a data bit with the power control bit. The receiving mobile typically responds to a turn down command by reducing its transmitter power by 1 dB and increases its power by 1 dB in response to a turn up command.
The disadvantage of the above described power control scheme is that the mobile transmitter signal is pulsed on and off when transmitting at less than the maximum data rate. While the system performs adequately with this scheme, it may cause interference to other electronic systems, such as hearing aids. The European radiotelephone system, Global System for Mobile communications, uses this power control scheme and exhibits such behavior. There is a resulting need for a power control scheme that enables the mobile to operate using a 100% duty cycle while providing fast and accurate closed loop power control from the base station to the mobile.